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><H1
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><A
NAME="LIBPQ-NOTIFY"
>31.7. Asynchronous Notification</A
></H1
><P
>   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> offers asynchronous notification
   via the <TT
CLASS="COMMAND"
>LISTEN</TT
> and <TT
CLASS="COMMAND"
>NOTIFY</TT
>
   commands.  A client session registers its interest in a particular
   notification channel with the <TT
CLASS="COMMAND"
>LISTEN</TT
> command (and
   can stop listening with the <TT
CLASS="COMMAND"
>UNLISTEN</TT
> command).  All
   sessions listening on a particular channel will be notified
   asynchronously when a <TT
CLASS="COMMAND"
>NOTIFY</TT
> command with that
   channel name is executed by any session. A <SPAN
CLASS="QUOTE"
>"payload"</SPAN
> string can
   be passed to communicate additional data to the listeners.
  </P
><P
>   <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> applications submit
   <TT
CLASS="COMMAND"
>LISTEN</TT
>, <TT
CLASS="COMMAND"
>UNLISTEN</TT
>,
   and <TT
CLASS="COMMAND"
>NOTIFY</TT
> commands as
   ordinary SQL commands.  The arrival of <TT
CLASS="COMMAND"
>NOTIFY</TT
>
   messages can subsequently be detected by calling
   <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>.
  </P
><P
>   The function <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> returns the next notification
   from a list of unhandled notification messages received from the server.
   It returns a null pointer if there are no pending notifications.  Once a
   notification is returned from <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>, it is considered
   handled and will be removed from the list of notifications.

</P><PRE
CLASS="SYNOPSIS"
>PGnotify *PQnotifies(PGconn *conn);

typedef struct pgNotify
{
    char *relname;              /* notification channel name */
    int  be_pid;                /* process ID of notifying server process */
    char *extra;                /* notification payload string */
} PGnotify;</PRE
><P>

   After processing a <TT
CLASS="STRUCTNAME"
>PGnotify</TT
> object returned
   by <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>, be sure to free it with
   <CODE
CLASS="FUNCTION"
>PQfreemem</CODE
>.  It is sufficient to free the
   <TT
CLASS="STRUCTNAME"
>PGnotify</TT
> pointer; the
   <TT
CLASS="STRUCTFIELD"
>relname</TT
> and <TT
CLASS="STRUCTFIELD"
>extra</TT
>
   fields do not represent separate allocations.  (The names of these fields
   are historical; in particular, channel names need not have anything to
   do with relation names.)
  </P
><P
>   <A
HREF="libpq-example.html#LIBPQ-EXAMPLE-2"
>Example 31-2</A
> gives a sample program that illustrates
   the use of asynchronous notification.
  </P
><P
>   <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> does not actually read data from the
   server; it just returns messages previously absorbed by another
   <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> function.  In prior releases of
   <SPAN
CLASS="APPLICATION"
>libpq</SPAN
>, the only way to ensure timely receipt
   of <TT
CLASS="COMMAND"
>NOTIFY</TT
> messages was to constantly submit commands, even
   empty ones, and then check <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> after each
   <CODE
CLASS="FUNCTION"
>PQexec</CODE
>.  While this still works, it is deprecated
   as a waste of processing power.
  </P
><P
>   A better way to check for <TT
CLASS="COMMAND"
>NOTIFY</TT
> messages when you have no
   useful commands to execute is to call
   <CODE
CLASS="FUNCTION"
>PQconsumeInput</CODE
>, then check
   <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>.  You can use
   <CODE
CLASS="FUNCTION"
>select()</CODE
> to wait for data to arrive from the
   server, thereby using no <ACRONYM
CLASS="ACRONYM"
>CPU</ACRONYM
> power unless there is
   something to do.  (See <CODE
CLASS="FUNCTION"
>PQsocket</CODE
> to obtain the file
   descriptor number to use with <CODE
CLASS="FUNCTION"
>select()</CODE
>.) Note that
   this will work OK whether you submit commands with
   <CODE
CLASS="FUNCTION"
>PQsendQuery</CODE
>/<CODE
CLASS="FUNCTION"
>PQgetResult</CODE
> or
   simply use <CODE
CLASS="FUNCTION"
>PQexec</CODE
>.  You should, however, remember
   to check <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> after each
   <CODE
CLASS="FUNCTION"
>PQgetResult</CODE
> or <CODE
CLASS="FUNCTION"
>PQexec</CODE
>, to
   see if any notifications came in during the processing of the command.
  </P
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